(Not Applicable)
(Not Applicable)
The present invention relates generally to the Z-pinning of composite material layers to each other, and more particularly to uniquely configured Z-pins for use in a Z-pinning process to achieve a mechanical lock between the layers and maximize the bond strength between the layers and the Z-pins.
As is known in the prior art, structural composites such as structural composite airframes typically comprise multiple composite stiffeners or stiffener members which are attached or fastened to prescribed areas of a composite substrate such as the aircraft skin. One such commonly used stiffener is referred to as a xe2x80x9chat stiffenerxe2x80x9d due to its shape. These composite hat stiffeners or other aircraft stiffeners are often used to reinforce thin composite structures such as wing and fuselage skins and bulkhead webs. In this respect, the purpose of such stiffeners is to supply rigidity and stiffness to the airframe of the aircraft as is required under certain flight load conditions. The stiffener must be attached to the skin in a manner wherein shear loads and out-of-plain loads due to peel (delamination) are transferred thereto.
Currently, numerous processes or techniques are employed to facilitate the attachment or fastening of a stiffener to an aircraft skin or other composite structure or substrate. More particularly, current practices include either co-curing, adhesively bonding, or mechanically fastening the stiffener to the skin. Though the co-curing and adhesive bonding techniques offer the minimum weight solution, the reliability of co-cured and adhesive bonded joints is generally low. Additionally, though mechanically fastened joints (e.g., bolts and rivets) have been successfully used in the prior art, the use of such mechanical joints requires that the laminate be reinforced so that it can react to bolt bearing loads. Further, the fasteners themselves are very heavy and expensive, with the cost of installation and inspection resulting in a high cost per fastener. As such, when one composite part is attached to another composite part through the use of bolts and/or rivets, such fasteners add weight, increase fabrication cost, and often contribute to local failure modes between the individual plies of the laminate composite.
In order to alleviate the above-described problems, a process referred to as xe2x80x9cZ-pinningxe2x80x9d has been developed in the prior art to facilitate the attachment of one composite layer or component, such as an aircraft stiffener or hat stiffener, to another composite layer or component/substrate, such as an aircraft skin. In the Z-pinning process, reinforcing pins or fibers are inserted at a radius region of the stiffener into the skin to increase the initial failure load of the joint between the stiffener and the skin, or through a flange portion of the stiffener and into the skin to resist crack propagation. In addition, the pins or fibers reinforce the individual plies of both the stiffener and the skin to resist delamination.
The insertion of the pins or fibers may be accomplished by softening the composite stiffener and skin, and thereafter driving multiple pins or fibers through the stiffener and into the skin through the use of, for example, an autoclave press, an ultrasonic transducer, or a hand-held pinning tool. When an ultrasonic transducer is employed, the resultant high frequency vibration of the pins or fibers results in the same being forced through the stiffener and into the skin. Irrespective of the particular device used to facilitate the insertion of the pins or fibers through the stiffener and into the skin, the pins or fibers are typically embedded in a transfer material for purposes of maintaining the same in prescribed orientations relative to each other and relative to that portion of the stiffener through which they are advanced. As indicated above, the advancement of the pins or fibers through the stiffener and into the skin effectively increases the initial failure load of the joint therebetween.
Though, as discussed above, the Z-pinning process provides advantages over the use of co-curing, adhesive bonding, or mechanical fastening, the pins or fibers themselves possess certain deficiencies which detract from their overall utility. More particularly, the prior art Z-pins are typically cylindrically configured with opposed beveled ends. The fibers, like the pins, also have generally circular cross-sectional configurations of substantially constant diameter. As will be recognized, in view of the shapes (i.e., uniform diameters) of the prior art Z-pins and fibers, they do not facilitate the mechanical lock or retention between the joined panels or layers (e.g., the stiffener and skin). Since they are of a constant diameter, only the bond strength between the composite material layers and the Z-pins or fibers themselves resists the movement of the layer(s) along an axis which is parallel or coaxially aligned with the axis of the Z-pin or fiber. Additionally, the shapes of such prior art Z-pins and fibers result in the same defining a relatively low surface area to which the composite materials of the stiffener and skin may be bonded. Due to the inability of the prior art Z-pins and fibers to mechanically lock the layers to each other, the integrity of the joint between the layers is dependent upon the strength of adhesion between the composite materials of the layers to the surface or bond area defined by the Z-pins or fibers, with a reduced surface area therefore resulting in an increased susceptibility to joint failure.
The present invention overcomes these deficiencies with prior art Z-pins and fibers used in a Z-pinning process by providing Z-pins which are uniquely configured to provide an increased surface or bond area over those known in the prior art, and further to facilitate the mechanical locking or retention of the joined composite material panels or layers to each other. As will be described below, the Z-pins of the present invention are provided with varying diameters, thus providing numerous advantages over the prior art Z-pins and fibers which, due to their constant or uniform diameters as explained above, do not provide the mechanical lock achieved by the Z-pins of the present invention or the enhanced bonding resulting from the increased surface area of the present Z-pins.
In accordance with a first embodiment of the present invention, there is provided a pin for Z-pinning at least two composite material layers to each other which comprises a generally circular first end having a first diameter and a generally circular second end having a second diameter. Extending between the first and second ends is a cylindrically configured central portion which defines at least two, and preferably three, radially extending flange sections which are spaced at substantially equidistant intervals between the first and second ends. The flange sections are each preferably of a diameter exceeding the larger of the first and second diameters. In the first embodiment, the first and second diameters are preferably substantially equal, as are the diameters of the flange sections which each preferably include an arcuately contoured peripheral edge. Additionally, the first and second ends and the central portion of the pin are preferably in coaxial alignment with each other. The pin may be fabricated from a metallic material or a composite material such as resin impregnated graphite or fiberglass materials.
In accordance with a second embodiment of the present invention, there is provided a pin for Z-pinning at least two composite material layers to each other which comprises a generally circular first end having a first diameter and a generally circular second end having a second diameter. Extending between the first and second ends is a central portion which defines a generally hyperboloidal outer surface having a maximum diameter which does not exceed the smaller of the first and second diameters. In the second embodiment, the first and second diameters are preferably substantially equal, with the first and second ends and the central portion preferably being in coaxial alignment with each other. The pin in the second embodiment may also be fabricated from a metallic material or a composite material such as a resin impregnated graphite or fiberglass material.
In accordance with a third embodiment of the present invention, there is provided a pin which constitutes a modified version of the pin of the first embodiment. The pin of the third embodiment comprises a generally oval first end having a first maximum width and a generally oval second end having a second maximum width. Extending between the first and second ends is a central portion which defines at least two, and preferably three, generally oval flange sections which are spaced at substantially equidistant intervals between the first and second ends and each have a maximum width exceeding the larger of the first and second maximum widths. The central portion of the pin defines an outer surface having at least one groove formed therein which extends from the first end to the second end.
In accordance with a fourth embodiment of the present invention, there is provided a pin which constitutes a modified version of the pin of the second embodiment. The pin of the fourth embodiment comprises a generally oval first end having a first maximum width and a generally oval second end having a second maximum width. Extending between the first and second ends is a central portion which defines a generally hyperboloidal outer surface having a maximum width which does not exceed the smaller of the first and second maximum widths. In the fourth embodiment, the outer surface of the central portion of the pin includes at least one groove formed therein which extends from the first end to the second end.